1. Field of the Invention
This invention relates in general to packoffs for casing hangers, and in particular to a metal packoff which will accommodate some movement between the casing hanger and the wellhead.
2. Description of the Prior Art
One type of oil and gas well assembly utilizes a wellhead at the surface which may be on the seafloor. A casing hanger secures to the upper end of a string of casing that extends into the well. The casing hanger lands in the wellhead to support the weight of the casing. An annular space exists between the exterior of the casing hanger and the bore of the wellhead. A packoff or seal locates in this annular space to seal against pressure from the annular space surrounding the casing below the casing hanger.
One type of prior art packoff is a metal seal. It is not subject to deterioration as much as elastomeric seals. One type of metal seal is generally U-shaped, with inner and outer walls separated by an annular cavity. An energizing ring is forced into this cavity to deform the inner and outer walls radially. This causes the inner and outer walls to seal tightly against the exterior of the casing hanger and the wellhead bore. Wickers, which are small circumferential parallel grooves, may be located in the wellhead bore and on the casing hanger exterior. The inner and outer walls embed into these wickers to provide the seal and to provide a gripping force for the seal member.
While this is workable, normally a casing hanger may move downward slightly relative to the wellhead after the packoff has been set. This occurs due to test pressure being applied to the casing hanger after the packoff is set. If such movement occurs, there is a possibility that the seal of the packoff may be lost due to this relative movement.